Patent Document 1 describes a parking assistance system that conducts the following: acquiring obstruction information using an ultrasonic sensor while a subject vehicle is moved before parked; accumulating the obstruction information in a vicinity-map producing unit to thereby produce a map in the vicinity of the subject vehicle; retrieving and determining a target parking region based on the vicinity map; computing a target movement path to the target parking region; and displaying on a display the target movement path to be superimposed on an image rearward of the subject vehicle.
Patent Document 1: JP-2003-54341 A
When a subject vehicle is to be parked in a parking space in a parking lot, a previously parked vehicle may be present in the vicinity of the parking space. Therefore, determining a target parking region where the subject vehicle is expected to be parked requires accurately detecting a position or a size of the previously parked vehicle.
However, detecting a neighboring vehicle using the ultrasonic sensor as described in Patent Document 1 does not result in accurately detecting the size or the like of the neighboring vehicle. The reason of this difficulty is as follows. First, an advance distance of the subject vehicle while the reflected waves are being received is regarded as a size of the parked vehicle. However, a corner of a vehicle is typically not right-angled but intricately-curved. An ultrasonic sensor of the subject vehicle thereby receives ultrasonic waves reflected from a part of the corner of the parked vehicle before the corner of the parked vehicle becomes situated perpendicularly to (or just in front of) the setting position of the ultrasonic sensor. Receiving the reflected waves is varied depending on a shape of the corner of the parked vehicle or a relative direction between the subject vehicle and the parked vehicle. Consequently, the advance distance while the reflected waves are being received does not simply correspond to the size of the parked vehicle to thereby tend to provide an improper parking assistance.